A doopboek is a baptismal register. Baptismal records are especially useful in the period before the introduction of the civil registration in 1811 (in most parts of the Netherlands). Baptismal records are often the earliest record created about our ancestors.
You may come across the term in finding aids and genealogical indexes, or on the covers of the imaged baptismal records.
See the article about baptismal records for more information on how to use and find these records.